1. Field of the Invention
This invention relates to a piezoelectric actuator having piezoelectric ceramics bonded onto a plate material and a method for manufacturing it, and more particularly to a piezoelectric actuator and a method for manufacuring it in which its response characteristic can be improved and its productivity can be improved by bonding the piezoelectric ceramics to the plate material based on an alloy forming reaction due to mutual diffusion phenomena.
2. Description of the Related Art
Conventionally, a piezoelectric actuator having piezoelectric ceramics formed on a plate material serves to generate on the piezoelectric ceramics the displacement which corresponds to supply voltage when voltage is supplied to the piezoelectric ceramics, so that the displacement of the piezoelectric ceramics causes the plate material to be displaced, or the deformation of the plate material is transferred to the piezoelectric ceramics, which are thus deformed, and the voltage corresponding to the displacement of the piezoelectric ceramics is generated on the piezoelectric ceramics. Such a piezoelectric actuator has been employed for a piezoelectric buzzer, an ultrasonic motor, an ink jet printer head, various kinds of sensors, etc.
There have been proposed various techniques for manufacturing a piezoelectric actuator such as a method for forming piezoelectric ceramics by providing particles in the form of paste, which become piezoelectric ceramics, on a plate material and then sintering them, and a method for bonding the sintered piezoelectric ceramics onto the plate material.
In the above mentioned method for providing the particles in the form of paste, which become piezoelectric ceramics, on the plate material and then sintering them, an electrode composed of conductive and heat resistant material such as platinum or palladium is formed on the plate material composed of ceramics or the like, which are a high-melting point material, and then, pasted particles which become piezoelectric, ceramics are provided on the electrode material, are sintered while holding them at the temperature of about 1000.degree. C. and the piezoelectric ceramics are formed on the plate material so that a piezoelectric actuator is manufactured.
As techniques for bonding the sintered piezoelectric ceramics onto the plate material, as disclosed in Japanese examined patent publication No. 64-87675, a method has been known in which the piezoelectric ceramics are bonded under pressure to the plate material made of metal by means of an adhesive agent produced by mixing metal powder of good conductivity with epoxy resin, acrylic resin, urethane resin, phenol resin or cyanoacrylate resin.
Further, a method has been also known, as disclosed in Japanese patent laid-open publication No. 62-277900, in which glass paste is applied to the surface of the plate material made of metal, the piezoelectric ceramics are provided thereon, and the glass paste is heated under the temperature of 450 to 600 C. so that the piezoelectric ceramics are bonded to the plate material made of metal.
When the piezoelectric actuator is manufactured in accordance with the method that the pasted particles, which serve as the piezoelectric ceramics, are provided on the above described plate material and then sintered, they are sintered under the temperature of approximately 1000 C., so that materials used for the plate material and the electrode are disadvatageously restricted.
Particularly, since the property of the plate material such as heat resistant temperature and coefficient of thermal expansion is greatly restricted, there have been found such problems that the material can be hardly selected freely and productivity is not good.
On the other hand, when the piezoelectric actuator is manufactured in accordance with the method that the sintered piezoelectric ceramics are bonded to the plate material, the piezoelectric ceramics do not need to be sintered after the piezoelectric ceramics are bonded to the plate material, so that the materials used for the plate material are not limited upon their selection. However, when the piezoelectric ceramics are bonded to the plate material so that the obtained product is employed as the piezoelectric actuator, they need to be strongly bonded to each other. Therefore, they must be bonded together in accordance with the above described means, that is, the above mentioned adhesive agent is used, or the glass paste is used.
However, since a resin exists in a bonding layer in accordance with the method in which the piezoelectric ceramics are bonded to the plate material by means of the adhesive agent, the bonding layer absorbs the displacement generated in the piezoelectric ceramics, which results in undesired deterioration in responsiveness. Specifically, in the case of high frequency oscillation, the absorption of the displacement is increased and the feature of the piezoelectric actuator is thus deteriorated. On the other hand, when the glass paste is used, a bonding layer between the piezoelectric ceramics and the plate material is composed of a glass material, so that the displacement of the piezoelectric ceramics absorbed by the bonding layer is lowered. However, since a heating process for heating the glass material is needed, a limitation is disadvantageously subjected to the property of materials used such as heat resistant temperaturte, coefficient of thermal expansion. Therefore, the degree of freedom for selecting materials is undesirably reduced and productivity is lowered.